Field of the Invention
The present invention relates to methods and apparatus for detecting faults in a two-wire power line. In particular, the present invention relates to detecting ground faults or leakage current in the two-wire train line cable on Electronically Controlled Pneumatic (ECP) equipped trains.
Description of Related Art
In the past, the railroads have typically operated trains having only a single air line extending the length of the train. This air line was used for both providing a source of compressed air and a medium for propagating braking signals. While this system has been used extensively in the past, it has several drawbacks. Signaling via air pressure messages propagating through the air line has a limited propagation speed.
In recent years, the Association of American Railroads (AAR) and individual railroads have investigated using ECP brake systems. These systems typically use electric messages on an electrical power line extending the length of the train to activate the brakes on each car because of the almost instantaneous propagation of the electric signal. Whereas, in a pneumatic pipe with numerous couplings, turns, and other restrictions, the pneumatic signal propagation is limited, the ECP brake system allows for nearly instantaneous activation of the railcar brakes along the entire length of the train.
ECP brake systems for railroads utilize a two-wire cable that carries both digital communication signals and system power. The system power used is presently 230 Volts DC, but other AC or DC voltages could be used. During start-up, the system is often powered to 24 Volts DC.
The North American Railroad Industry, led by the AAR, has developed standards for ECP brakes. These standards include an electric two-conductor train line cable, which operates at 230 VDC, for providing power and communications to and from the ECP units on each car.
The two-wire power system is floating relative to earth ground. Typically, this two-wire power system is powered from one or more locomotive batteries through a DC to DC converter. The design of the system is such that these two conductors are isolated from ground.
U.S. Pat. No. 6,127,747 entitled “Method and Apparatus for Detecting Leakage Current on a Two Wire DC or AC Power Line” and U.S. Pat. No. 6,217,126 entitled “Railway Emulation Brake” both relate to ECP brake systems and are assigned to the same assignee as this application.